This can have several causes. 1) If the fan in the freezer is working (it will usually turn off when you open the door), then it is usually a defroster issue. If the fan is dead, then it's the fan.
Assuming it's the defroster, and the refrig is old enough, or inexpensive enough to not have any computers or fancy displays, the it is usually the defrost timer that is bad. Depending on the model, the timer is usually underneath the front kick panel, or more often near where the knobs are that control the temperature. Try finding the defrost timer - it may be under a removable plastic panel where the knobs are or where the light bulb is, and if you do, there is usually a knob on it that you can turn with a short, wide screwdriver or coin. Turn it until the refrig turns off and this will start the defrost cycle. It may take a while to melt the ice. The ice is under the bottom or back panel in the freezer. You can remove the panel with a couple screws and melt the ice with a hair dryer also. Assuming this was the problem - that the timer is no longer working, you will have to turn the timer again to start the refrig back up, unless moving it got it running again. By defrosting it manually, you have bought yourself a couple days before it ices up again, so go and buy a new timer. Occasionally, the problem could also be that the heater coil is shot, or the wire to it has rusted off, or a small temperature sensor attached near the coil is bad, but it is usually either the fan or the timer. By the way, there's usually a wiring diagram on a folded piece of paper hidden somewhere behind the kick panel. Good luck.

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defrost and clean the entire fridge/freezer and clean out the grill and all the back and bottom under the fridge...with a vacuum..alot of times the fins in the back become coated in dust and cannot effectively cool the appliance..worth a try

check the freezer evaporator thermistor on the top left corner clipped to the evaporator,if its not 16k ohm when at 32 degrees in a glass of ice water after 4 minutes it will defrost too many times and cause this issue,you need to unplug the J1 connector on the mother board in the rear behind the cover and check terminals 4 & 5 un the plug,not on the board though

You have something wrong with the vent going from the freezer to the fridge. You probably have a defrost problem and ice is blocking the vent that goes from the freezer and comes out in the roof of the rerigerator compartment. Replace your defrost timer and bimetal.

It's likely the door was inadvertently left open, and the cold air passages from the freezer have built up enough ice that the cold air from the freezer has no way to enter the refrigerator. Check the cealing of the refrigerator. If you can see or feel ice in the passages between the freezer and fridge, you can probably solve the problem by finding a safe temporary home for your food, and leaving the refrigerator unplugged overnight.

Check the bottom refrigerator door gasket for any possible spilled food or drink that may have leaked onto it. Clean this area with warm soapy water. It may solve your problem. Also check under the drawers and make sure there is nothing there. If there is a drain under the drawers clean it and put some hot water with bleach into it to flush it out.